This invention relates to a method of preparing aromatic polyimides which have perfluoroalkyl groups and are soluble in various organic solvents and varnish compositions using the obtained polyimides.
As synthetic resins aromatic polyimides are excellent in heat resistance as well as mechanical and electrical characteristics and have been widely used as industrial materials for various articles including electric and electronic devices.
Aromatic polyimides hitherto developed are hardly soluble in ordinary solvents except sulfuric acid. Therefore, it is practically impossible to use an aromatic polyimide solution for casting and coating purposes. A substitutive method is using a solution of an aromatic polyamic acid, which is the precursor of a desired polyimide, in an organic solvent. After casting the polyamic acid solution into a desired form or coating the solution on a substrate or base member the polyamic acid can be converted into the intended polyimide by a cyclization reaction. However, the application of this method is limited because a heat treatment at a temperature as high as 250.degree.-400.degree. C. is necessary for the cyclization reaction.
JP-A No. 60-49030 shows that aromatic polyimides characterized by a bulky structural unit having tetraphenylthiophene group are soluble in m-cresol and some other organic solvents. However, m-cresol is a solvent inconvenient for industrial handling, and the disclosed polyimides do not exhibit high solubilities in other organic solvents. JP-A No. 62-292836 shows preparing a soluble polyimide by carrying out the reaction of an aromatic diamine with pyromellitic dianhydride in a phenolic solvent. However, the obtained polyimide is not soluble in many kinds of organic solvents, though it is soluble in o-chlorophenol.
U.S. Pat. No. 3,179,630 shows preparing a polyimide by reacting an aromatic diamine with an aromatic tetracarboxylic acid dianhydride in an organic solvent in the presence of a lower monocarboxylic acid anhydride and a tertiary amine. This patent relates to aromatic polyimides not containing fluorine, and the obtained polyimides are insoluble in organic solvents.